Atomic models and the periodic table
Ideas about the atom changed with new experiments. Dalton saw it as a tiny solid ball. Thomson found electrons inside it (a ball with small negative bits). Rutherford shot particles at gold foil and showed most of the atom is empty, with a small heavy nucleus in the middle. Bohr placed electrons in fixed shells. Today we say electrons live in cloud-like regions called orbitals, but the shell picture still works well for the first 18 elements.
The atomic number Z is the number of protons. It is also the number of electrons in a neutral atom. Shells hold 2, 8, 8 electrons (first three). The periodic table puts atoms in order of Z. A row is a period (= number of shells). A column is a group; atoms in one group have the same number of outer electrons, so they act alike. Going across a period, atoms change from metals to non-metals, and the last group (noble gases such as neon) has full shells and hardly reacts.
Chemical bonds: ionic and covalent
Atoms join to get a full outer shell (usually 8 electrons). There are two main ways.
Ionic bond: a metal gives electrons and a non-metal takes them. Na gives 1 electron to Cl and becomes Na⁺; Cl becomes Cl⁻. These ions have opposite charges and stick together in a regular pattern called a crystal lattice. Ionic solids have high melting points and conduct electricity when melted or dissolved.
Covalent bond: two non-metals share electron pairs. In water, O shares one pair with each H, so each atom gets a full shell. Covalent molecules (water, CO₂, methane) usually have low melting points and do not conduct. A shared pair can be single (one pair), double (two pairs) or triple (three pairs).
Inorganic nomenclature: naming compounds
Names follow simple rules so everyone knows the compound from the name.
- Ionic, simple: metal name, then non-metal with -ide. NaCl = sodium chloride; MgO = magnesium oxide.
- Formula by crossing: write the ions with charges, swap the numbers, simplify. Al³⁺ and O²⁻ give Al₂O₃ (aluminium oxide). The total charge must be zero.
- Metals with more than one charge: add a roman number. FeCl₂ = iron(II) chloride, FeCl₃ = iron(III) chloride.
- Group ions: OH⁻ hydroxide, NO₃⁻ nitrate, SO₄²⁻ sulfate, CO₃²⁻ carbonate. Ca(OH)₂ is calcium hydroxide.
- Two non-metals: use number prefixes: mono-, di-, tri-, tetra-. CO = carbon monoxide, CO₂ = carbon dioxide.
- Acids: HCl is hydrochloric acid, H₂SO₄ is sulfuric acid, HNO₃ is nitric acid.
Basic organic chemistry
Organic chemistry is the chemistry of carbon compounds. Carbon has 4 outer electrons, so it makes 4 bonds. It can join to other carbons in chains, branches and rings (this is called catenation), and it makes single, double and triple bonds.
Hydrocarbons contain only C and H. Alkanes have single bonds only: CₙH₂ₙ₊₂ (methane CH₄, ethane C₂H₆, propane C₃H₈). Alkenes have one C=C double bond: CₙH₂ₙ (ethene C₂H₄). Name roots: 1 C = meth, 2 = eth, 3 = prop, 4 = but, 5 = pent, 6 = hex. A functional group gives the molecule its character: the –OH group makes an alcohol such as ethanol, C₂H₅OH, used as solvent and in sanitisers.
Organic molecules are everywhere: fuels, wax, plastics, dyes, oils and the binders in paints.
Try it
Kitchen test: dissolve salt in water and a little sugar in another glass. Salt (ionic) breaks into ions; sugar (covalent) stays as molecules. Then in the 3D: (1) set the slider to Z = 17 and read the outer electrons; (2) before you tap, predict the formula of calcium nitride, then check it with the pickers; (3) count the bonds on each carbon in all four molecules. The answer is always 4.
Key formulas and definitions
- Atomic number Z = number of protons = number of electrons (neutral atom)
- Shell capacity (first three shells): 2, 8, 8
- Ionic formula: cross the charge numbers, then simplify
- Total positive charge = total negative charge
- Alkane: CₙH₂ₙ₊₂, Alkene: CₙH₂ₙ
- Name roots: meth 1, eth 2, prop 3, but 4, pent 5, hex 6
Worked examples
1. Write the electron arrangement of chlorine (Z = 17). Which period and group?
17 = 2 + 8 + 7, so shells 2, 8, 7. Three shells, so period 3. 7 outer electrons, so group 17. It needs 1 more electron, so it forms Cl⁻.
2. Write the formula of aluminium oxide.
Ions: Al³⁺ and O²⁻. Swap the numbers: Al₂O₃. Check: 2 × (+3) = +6 and 3 × (−2) = −6. Total is zero.
3. Name CaCl₂ and FeCl₃.
CaCl₂ = calcium chloride. FeCl₃: three Cl⁻ means iron is Fe³⁺, so iron(III) chloride.
4. Is MgO ionic or covalent? What about CO₂?
MgO is a metal plus a non-metal. Mg gives 2 electrons to O, so it is ionic. CO₂ has two non-metals that share electrons, so it is covalent.
5. How many hydrogen atoms in propane? Write its formula.
Propane has 3 carbons. H = 2n + 2 = 2 × 3 + 2 = 8. Formula C₃H₈.
6. An element has atomic number 15. Give shells, period, group and the ion it forms.
15 = 2 + 8 + 5. Period 3, group 15, 5 outer electrons. It needs 3 more to fill 8, so it can form P³⁻ or share 3 electrons in covalent bonds.
Common mistakes
- Saying electrons are in the nucleus. The nucleus has protons and neutrons only.
- Forgetting that the total charge must be zero when writing ionic formulas.
- Writing "sodium chlorine" instead of "sodium chloride". The non-metal ends with -ide.
- Thinking carbon makes 2 or 3 bonds in organic molecules. It makes 4 each time (a double bond counts as 2).